This invention relates to a display device, such as a liquid crystal display device or the like, that comprises a reflective holographic optical element for ambient light illumination. More particularly, this invention relates to such display device that also includes a fluorescent element to increase the light redirected by the reflective holographic optical element and thereby enhance the brightness of the display.
A simple display combines dark and bright areas to create a desired image. For example, the dark areas may form alphanumeric characters surrounded by bright areas. The common example is produced by a liquid crystal display device and is found in calculators, watches, laptop computers and the like. The device comprises a liquid crystal panel that contains a layer of liquid crystal material sandwiched between front and back polarizers having perpendicular axis of polarization. Electrodes adjacent to the liquid crystal layer apply a localized electric field to selected regions of the layer. In the absence of an electric field, polarized light is admitted through one polarizer, reoriented by the liquid crystal layer and passes through the opposite polarizer, so that the panel appears transparent. However, an electric field disrupts the liquid crystal material to prevent light from being reoriented to pass through the opposite polarizer, resulting in an opaque region. By selectively applying electric voltage to the electrodes, individual regions of the panel are switched between a transparent state and an opaque state to create a desired display.
The liquid crystal panel does not generate the light needed for viewing the display. For this purpose, it is known to combine the liquid crystal panel with a reflector facing the back of the panel. Ambient light passing through a transparent region of the liquid crystal panel is reflected and is retransmitted through the liquid crystal panel to form a bright area for the display. Of course, light is not reflected through an opaque region.
It has been proposed to provide ambient light illumination of a display by a reflective holographic optical element, also referred to as a holographic reflector. In contrast to reflection by a mirror, a holographic element acts as a diffraction grating to redirect light in an interference pattern. One advantage of a holographic reflector is that light illuminating the surface at angles outside the interference pattern is redirected within the pattern to enhance the brightness of the display at preferred viewing angles. However, because it produces an interference pattern, the reflective holographic element tends to redirect light within a relatively narrow spectral band. It is desired to capture ambient light that is outside the effective spectral band of the holographic element and to redirect the light within the reflection pattern of the holographic element, so as to increase the brightness of the display.